(1) Field of the Invention
This application is directed to improvements in rolling mill housings used in rolling operations in the flat rolled metal industry. In particular, the present invention is directed toward a multi-roll cluster type of rolling mill.
(2) Description of Related Art
Cluster mills are popular in the rolling mill industry when a high gauge reduction is taken, a thin exit gauge is rolled, or a combination of the two. A cluster mill provides many advantages to the operation of a rolling mill and includes the following: small diameter work rolls, high housing stiffness, and a simplified gauge control. In many previous applications, the cluster mill housing has been built based on a mono block design, such as seen and described in U.S. Pat. No. 5,421,184, U.S. Pat. No. 2,187,250 in FIG. 8, and U.S. Pat. No. 2,776,586 in FIG. 8.
In particular, the centerline gauge (or thickness) control is excellent due to the high mill stiffness where any entry gauge increase is immediately met with a higher rolling force. The gauge control is very simple and supplied by rotating eccentric bearings on a support roll to adjust the roll gap. The developed rolling force is transferred to the mono block housing through the roll saddles at various angles which add to the mill stiffness. The rolling force is not thereby transferred into the mono block housing in the vertical direction only.
Though a Cluster mill has historically been attractive for many rolling applications, there is a need for improved flexibility in the rolling operation. One disadvantage to using a Cluster mill is a very small roll gap opening when there is a strip breakage. After a strip break, the improperly rolled metal strip is called a cobble. In many cases, a cobble results in many pieces of metal strip remaining within the mono block, and pieces of the cobble wrap around various rolls in the cluster roll arrangement. A cobble is a common, though infrequent, event during the rolling operation. Depending upon how quickly the entry side metal strip can be stopped, there may be damage to the rolls and ancillary equipment with a significant amount of metal strip to remove. Removal can take from several minutes to several hours depending upon the extent of damage to the rolls and other equipment. Sometimes, it is very difficult to remove the cluster mill rolls from the mono block due to jamming from broken strip. The ability to open the work rolls to a wide gap quickly in the event of a sudden strip tension loss, which indicates a strip break, would greatly help prevent cobbles from causing rolling mill damage. The desired opening gap to minimize damage is higher than currently available with the mono block work roll movement.
In addition, the Cluster mill has a limited range of work roll diameters that will operate within the design of the mono block. This lowers economic appeal. Work rolls are normally surface refinished by regrinding when they are worn out, and a limiting operating range makes reuse by grinding very limited.
Another disadvantage of the Cluster mill is the reduced ability to be flexible for a varied rolling operation. It is highly desirable in some commercial settings to have a single rolling mill capable of cold rolling with a heavy reduction and temper rolling with a light reduction. A temper rolling configuration preferably utilizes a larger work roll size. Larger work rolls allow for a longer work roll life, a faster rolling operation, favorable strip shape, and better rolling feasibility. In contrast, the mono block Cluster mill is unattractive for a mill that is capable of both temper and cold rolling operations. In particular, the small work roll diameter range is unsuitable for a mill configured to do both types of rolling.
The mono block design has a poor ability to thread the mill due to the small roll opening. It is difficult for the beginning end of the strip to always be flat and suitably ready to conveniently enter a small roll gap. The strip may be reluctant to enter the roll gap bite due to minor entry strip bending issues and require the manual intervention of an operator with long handled manual tools.
In a mono block design it is difficult to determine the rolling force, i.e. the vertical separation force, between the two work rolls during the rolling operation. The rolls are positioned in the rolling housing so that the vertical rolling force is dispersed into the mono block by several rolls. This highly restricts the ability to measure the rolling force with accuracy. It is desirable to measure the rolling force and use it to improve yield by more accurate rolling to the correct gauge in the initial setup.
The mono block is not designed for a convenient and accurate tilting arrangement when there is a significant side to side gauge variance in the metal strip, that is, a wedge shaped strip. Depending upon the upstream hot rolling operation, a metal strip will often have a moderate thickening in the middle of 1 to 5% of the nominal gauge. After hot rolling, the strip may be slit into two halves (or more) for further downstream processing which includes rolling on a Cluster mill. This presents a wedge shaped strip to the Cluster mill with an unpredictable thickness across the width. Since the mono block does not have a rolling force measurement, it is difficult to make an accurate side to side rolling gap correction. The rotation of the crown eccentric rings used for profile control do not provide enough tilting capability. Consequently, a wedge shaped strip will have other problems in rolling which include strip breakage, creating camber, creating centerbuckle, creating uneven edge wave, and creating other unusual strip flatness problems.
Others have recognized operational problems of the mono block design and attempted improvements. For example, U.S. Pat. No. 5,857,372 describes a split housing and prestress rod arrangement with the goal of improving various operational problems. The methods utilized are mechanically complicated, expensive to machine, and do not allow for the rapid roll opening needed to prevent damage when the strip breaks. The design does not consider tilting of the mill. Also, the ability to adjust passline is very restricted and is equivalent to a mono block design.
U.S. Pat. No. 5,996,388 considers the use of hydraulic cylinders to prestress a rolling cage useful in a hot bar rolling operation. The design is unsuitable for a high mill stiffness to take advantage of a simplified, satisfactory commercial gauge control system in a flat rolled product. The methods utilized are mechanically complicated, expensive to machine, and do not allow for the rapid roll opening needed to prevent damage when the strip breaks. The design does not consider tilting of the mill or passline adjustment.
U.S. Pat. No. 6,260,397 considers the need to provide operational improvements that are not available with a mono block. The design does not take advantage of the mono block stiffness, but rather adds an additional pair of larger mill housings which greatly adds to the expense of the mill. The design does not use the simplified gauge control available with a mono block, and is not a prestress design. The design has a relatively low mill stiffness which requires a complicated gauge control system.